R. M. SKOMOROSKI AND A. SCHRIESHEIM
1340
and MoFBin the final state. The coefficients, as functions of composition, at 25' were
+ +
p = 0.000801 (1 6.99s (bE/dP)T = -1.780 (1 3.905s
-+ 6.07s*) atm.-l
(4)
mole-'
(5)
+ 2 . 5 8 5 ~ ~cal. ) atm.-'
where z represents the mole fraction of MoFBin the gaseous mixture. All other corrections to standard states were negligible. AEcO/;l.lis just the sum of items 4 through 10 divided by the mass of molybdenum reacted. Derived Data.-The following data were derived for the formation of molybdenum hexafluoride gas a t 25" Energy of formation AEf" = AEc" = -371.17 f 0.2* kcal. mole-' Heat of formation AHf" =
Entropy of formation ASfo =
-372.36 =t0.22 kcal. mole-'
-
72.13 cal. deg.-l mole-'
(Gibbs) free energy of formation AFf" = -350.83 =k 0.22 kcal. mole-'
The atomic weight of molybdenum22 was taken as 95.95 g. (g.-atom)-l. The entropies, So, a t 25O, of M o ( c ) , ~F2(g)16 ~ and MoF6(g)16 were taken as
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6.83, 48.45 and 80.01 cal. deg.-' mole-', respectively. The uncertainties given in these estimates are uncertainty intervals23equal to twice the overall standard deviation arising from known sources. Conclusion The standard heat of formation of molybdenum hexafluoride gas has been determined to be -372.35 f 0.22 kcal. mole-' by direct combination of the elements in a bomb calorimeter. h'o significant trend in the final results was observed although the initial fluorine pressure in the bomb and the extent of certain (minor) side reactions varied in the course of the experiments. The value derived by the direct reaction is believed to be more reliable than a value of -382 f 4 kcal. mole-' obtained by Myers and Brady5 from the heats of hydrolysis and solution of Mop6(]),Moos and XaF in dilute sodium hydroxide. Further work is required to shed light on the source of the discrepancy. Acknowledgments.-The assistance of Messrs. R. W. Bane, J. F. Goleb, B. D. Holt and R. E'. Larsen, who provided the special analyses required, is gratefully acknowledged. (22) E. Wichers, J . Am. Chem. Soc., 80, 4121 (1958). (23) F. D. Rossini, Chapter 14, "Experimental Thermochemistry," F. D. Rossini, Editor. Interscience Publishers, Inc., New York, N. Y., 1958, pp. 297-320.
THE HYDROGENATION OF PYRIDINE IN ACID MEDL4 BYR. M. SKOMOROSKI AND A. SCHRIESHEIM Esso Research & Engineering Company, Linden, New Jersey Received February 6,1961
Pyridine hydrogenation was studied in aqueous solutions of acetic and sulfuric acids using platinum dioxide catalyst a t 25 f 2' and 21 atm. hydrogen pressure. The purpose of this research was to investigate the role of the medium in hydrogenation reactions involving nitrogen compounds. It was found that the acid medium profoundly influenced the course of the reaction. With both acids the rate of reaction was found to vary as a function of the acid to pyridine mole ratio. Maxima in the rate were discovered a t acid to pyridine mole ratios of 0.5-1.5 and a t higher ratios the rates decreased. The data may be interpreted on the basis of a reduction of pyridinium ions rather than pyridine.
Introduction The hydrogenation of basic compounds, such as pyridine and its alkyl substituted derivatives, is known to occur readily in acid media over platinum oxide However, the choice of solvent can influence the hydrogenation rate markedly. For example, pyridine alone in most solvents poisons platinum oxide catalyst and no reduction takes place a t 25' and 2-3 atmospheres pressure, but in glacial acetic acid it is reduced to piperidine easily a t these conditions. Also pyridine hydrochloride and pyridinium salts are reduced more readily than pyridine in absolute alcohol.' However, no extensive work appears to have been done on pyridine hydrogenation in media of different acidity. The purpose of this research was to investigate in some detail the mechanism of pyridine hydro(1) T. S. Hamilton and R. Adams, J. Am. Chem. Soc., SO, 2260 ( 1928).
( 2 ) P. H. Emmett, "Catalysis," Chap. 4, Reinhold Publ. Corp., Piew York. N. Y.,1957. (3) H. Gilman, "Organic Chemistry," Chap. 9, 2nd Ed., John Wiley
and Sons. Inc., New York, N. Y., 1948.
genation in acid media. Accordingly, pyridine hydrogenation was studied in aqueous solutions of acetic and sulfuric acids using platinum dioxide catalyst. Also, experiments have been made in other solvents using pyridine or benzene for comparison purposes. Experimental Materials.-The platinum dioxide catalyst (84.12% Pt) used in this study was obtained from the Baker Catalysts Co., Inc., and used aa received without further treatment. Standard BET measurements using nitrogen adsorption gave 54 m.*/g. for catalyst surface area, 0.15 cc. average pore volume and 58 A. average pore radius. The catalyst particle size distribution (in microns) as determined by standard size screen sieves was: (8%) > 250, (19%) 250177, (49%) 177-149, (24%)
